Wednesday, March 9, 2011

Imagining Cormac's Mill,

What might Ireland's first Mill have looked like?

The Hill of Tara
In a previous article we explored the very real possibility that Cormac's Mill, reputedly the first mill in Ireland, was located on the slopes of the Hill of Tara.

The Mill was constructed in the 3rd Century at the request of Cormac Mac Airt, King of Tara, and I believe it was located at a site beside Tara Hall at the place where the Nith stream descends into the Nith Glen.  The evidence from the historical documents certainly suggests this, but is it possible? Was the site suitable for a mill?  What would it have looked like and how would it have operated?

In order to explore these questions we first have to look at the milling technology of the early Medieval period.  In their book 'A New History of Ireland, Prehistoric and early Ireland', Dáibhí Ó Cróinín and Theodore William Moody set out the milling technology that was becoming common in Ireland by the 6th and 7th Century.

Figure (i). A description of the early medieval horizontal millwheel.

These early mills were operated by impounding a stream or spring with a dam and releasing the water down a flume to turn the mill wheel that directly turned the mill stone.  I've made a sketch of the arrangement, Fig (ii).  It's interesting to note that these wheels were remarkably similar to modern day Turgo wheels, and later developments in horizontal millwheels refined the principle even further. 
Figure (ii). Horizontal Waterwheel.

There is a wonderful description of the eight parts of the horizontal mill in PV Wright's book 'A Social History of Ancient Ireland', pg 335, and also an illustration of how established the technology had become by the way in which the Brehon Laws were written to facilitate their construction,  safeguard the investment and to protect the workers from injury.

To date, the earliest horizontal mill found in Ireland has been dated to circa 630 AD, about 250 years after Cormac's Mill.  Is it really possible that the technology was in use so much earlier?  Given that the horizontal waterwheel had been described in Greek texts in the 1st Century BC it is entirely probable that the technology reached Ireland long before the 6th Century.

So, if Cormac's Mill was located at Tara Hall, how would it have operated?   How much of a resource was available, and how much was required?  Please suspend your disbelief while we make a long string of assumptions!

First we'll make some assumptions about how much power was required to operate the Mill.  Two horsepower (HP) would have been sufficient to spin the millstone, and with an efficiency of 50% and a head of 3.5m, the mill would have required a flow of 100 litres/second.

It's difficult to know precisely what the climate was like at the time, but it is probably fair enough to assume it was broadly similar to today.  For the sake of this examination we'll assume an annual rainfall of 935mm across the catchment and an evapotranspiation loss of 55%.

The catchment of the Nith Stream is quite small at Tara Hall (1.6km²), but there are a number of significant springs that are fed from the aquifer beneath.  It's impossible to know how much these springs contribute to the flow without gauging them, but it is worth noting that the early historic texts say that the spring 'Nemhnach' supplied the water for the mill.  While it is very unlikely that the spring alone supplied enough water for the mill, this reference does strongly point towards the mill operating with a mill pond, as was typical of most early medieval mills.  We'll assume a constant contributory flow of 4 litres per second from all the springs in the catchment. 

Having constructed a flow duration curve for the Nith Stream and assuming a large mill pond capable of holding 4000m³ of water (2 acres, 50cm deep), we can now construct a statistical graph to illustrate just how many hours of milling might have been possible at this site per day. 

Fig (iii).  Assumed hours of production.

The chart in Fig (iii) is in the form of a duration curve and can be read as showing how many days of a given production or more may have been possible for the mill.  For example; for 164 days of the year 5 hours or more of constant milling was possible.  It can also be read as showing that for  201 days (365-164), no more than 5 hours of milling was possible per day.

I think the assumptions I have made are realistic enough not to render my argument worthless, and I hope I have raised the possibility that Cormac's Mill was located on the Nith Stream at Tara Hall, and that it was a very early Horizontal Millwheel.  Clearly there was enough flow in the stream to fill a mill pond and supply enough power to operate the mill.

Anyway, thanks for reading, and I'll develop this section as I get a chance to research the topic further.  If you are curious about the potential for Hydropower at any location in the country, why not check out the site survey at  It's a free service.